Multibeam optical instruments generally include a primary mirror situated in the optical instrument, a central secondary mirror positioned facing the primary mirror, and a plurality of peripheral mirrors reflecting light towards the central secondary mirror. Each of these peripheral mirrors needs to be protected against direct entry of sunlight. Protection prevents optical disturbance and enables regulation of the temperature in the vicinity of the focal planes. This protection assures the optical performance of the instruments.
These instruments mounted on satellites are placed in orbit by launch vehicles. The small volume allocated under the nosecap of the launch vehicle makes it impossible to have a fixed protection device disposed in front of the optical instrument. It is therefore necessary to deploy this protection in flight, before the operational phase.
The technical problems encountered in deploying large structures are principally:
In the stowed configuration: restricted volume for stowing the structure, maintaining the integrity of that structure in the folded configuration, in the face of mechanical and thermal attack caused by launching the spacecraft (notably non-deterioration of the very fragile thermal protection elements).
During deployment: control of deployment in kinematic terms, regulation of speed in order to prevent shocks at the end of deployment.
In the deployed configuration: assuring stability and stiffness in flight in order to guarantee the controllability of the craft and to assure correct positioning of the structure in order to assure the limitation of entry of sunlight and no blocking of the field of view.
FIG. 1 represents a prior art satellite optical instrument protection device. That device includes a plane solar screen 101 positioned at a certain distance from the satellite 100 in such a manner as to protect the optical instrument against stray light coming from the sun 103. On some missions, the use of a plane screen implies having a rotary element (the solar screen) of large size. This solution also gives rise to problems: reliability problems, attitude orbit control system (AOCS) disturbance problems, problems of disturbance at the optical measurement level, and service life problems resulting from the mechanical elements employed.
Moreover, to prevent entry of sunlight during certain periods, complementary pivoting of the solar screen is necessary, which makes this type of solution more complex.
Thus protection close to the optical beams is a more efficacious solution because it requires no movement throughout the operational phase.
To provide this type of enclosing protection, it is possible to use several technologies based on a deployable support structure retaining and positioning a flexible substrate.
The support structure may be inflatable and stiffenable in flight but control of deployment is complex because it is difficult to model by calculation and complex to test. Moreover, stiffening methods are irreversible and therefore do not enable ground testing of a design intended to fly.
Known already are flexible protection devices based on the use of a flexible cylindrical sheath composed of thermal protection foil. This element is folded on itself during the stowage phase and then deployed and tensioned. This solution has a number of drawbacks. First of all, it is difficult in stowage to prevent deterioration of the membranes constituting the thermal protection blanket. These are very fragile and are sensitive to mechanical attack. The degraded membrane produces dust which, on deployment, is dispersed over the instrument and degrades its optical performance. Moreover, to obtain compact stowage, it is necessary to fold the flexible elements, and this folding is generally irreversible and weakens the membrane. Finally, the energy necessary to tension the flexible element in order to ensure its correct final positioning is difficult to calculate because in this case deployment is not particularly reproducible. This results in an overestimation of the energy needed for tensioning the flexible element in order to ensure its correct final positioning, which may cause deterioration of some components and the membrane.